Differentiation and determination of pearlite type structure in cold heading steel wire rod

The pearlite type structure in steel (referred to as pearlite) generally includes three types: lamellar pearlite, sorbite, and troostite, and they usually present a lamellar structure. There are indeed confusions and misunderstandings in how to clearly distinguish these three types of organizations in production practice. We have done some more detailed work to discuss with you.

1. Basic concepts about pearlite

1.1 Lamellar spacing of pearlite

The austenite of the eutectoid in the cold heading steel wire rod will decompose into a mixture of ferrite and cementite when it is cooled below the critical point A1, called pearlite, and the pearlite obtained by slow cooling becomes flakes Shape, called flake pearlite. The areas in the lamellar pearlite with roughly the same lamellar direction are called pearlite clusters. In an austenite grain, several pearlite clusters can be formed. The (vertical) distance between the centers of two adjacent cementite (or ferrite) sheets in the pearlite cluster is called the pearlite sheet spacing. The size of the lamella spacing is mainly determined by the formation temperature of pearlite. As the cooling rate increases, the temperature at which austenite transforms into pearlite gradually decreases, that is, the degree of supercooling during the transformation continues to increase, and the resulting pearlite is transformed The chip pitch is also continuously decreasing.

Generally, the so-called flaky pearlite has a sheet spacing of about 150-450nm; the sorbite sheet spacing is about 80-150nm; the sheet-shaped pearlite formed at a lower temperature with a sheet spacing of 30-80nm is in production The above is called troostite.

The specific formation temperature range of pearlite type structure is: pearlite is the critical point A1～650℃; sorbite is 650～600℃; troostite is 600～550℃.

Actually, there are also different divisions on the value of the interlamellar spacing of the pearlite type structure. For example, some data in the literature are pearlite: greater than 0.4; sorbite: 0.2～0.4; troostite: less 0.2; In addition, coarse pearlite: 0.6 to 0.7; pearlite: 0.35 to 0.5; sorbite: 0.25 to 0.3. It is also believed that: the pearlite type structure with a lamellar spacing of about 0.1, 0.25, 0.6 is troostite, sorbite, and lamellar pearlite, respectively.

As for the confusion of the range of pearlite layer spacing, it can be clarified according to the relationship between organization and performance. Since 150nm corresponds to a turning point in the performance of pearlite, it is reasonable to believe that the sheet spacing of the so-called flaky pearlite is about 150-450nm; the sheet spacing of sorbite is about 80-150nm; troostite It is more reasonable to divide the chip pitch of 30-80nm.

1.2 Pearlite in optical microscope

Generally, the so-called flaky pearlite refers to the lamella pearlite that can be clearly distinguished under an optical microscope (usually 500 times observation conditions); if the distance between the pearlite sheets is so small that the light mirror is difficult to distinguish, this This kind of flake pearlite is called sorbite. In fact, when observed with an electron microscope, whether it is sorbite or troostite formed at a lower temperature, it is a lamellar structure, but the distance between the plates is different. Different literatures have basically the same description of the magnification of the optical microscope on the ability to distinguish sorbite. On the basis of satisfying the corresponding numerical aperture, it is believed that under the condition of 400 to 500 times, the flake pearlite can be distinguished, 800 to 1000 times. Can distinguish sorbite. According to the GB/T13298-1991 standard, usually distinguish pearlite and troostite are observed under 500 times magnification. The approximate judgment is: if the magnification is 500 times, ferrite and cementite are difficult to distinguish between Soxhlet Body type pearlite.

However, we believe that there is a need to explore the distinction between lamellar pearlite and sorbite based on whether the lamellar structure can be distinguished in an optical microscope.